Method of protecting magnesium and its alloys from corrosion



Patented Mar. 16, 1943 METHOD OF I ROTECTING MAGNESIUM AND ITS ALLOYS FROM CORROSION William S. Loose, Midland, Mich., assignor to The Dow Chemical Company, Midland, Micln, a corporation of Michigan 1 No Drawing. Application November 14, 1938,

Serial No. 240,344

2 Claims. (Cl. 204-56) The invention'relates to methods of providing corrosion resistant coatings for magnesium and its alloys. One of the objects of the invention is to provide a method of developing a corrosion resistant film-like coating upon magnesium and its alloys. Another object is to provide a method of the aforesaid character whereby articles ,of magnesium and its alloys may more readily retain paint, varnish, and the like.

Heretofore it has been proposed to subject articles of magnesium and its alloys to the chemical action of an aqueous solution of hydrofluoric acid or the water-soluble fluoride salts by immersing the article in such solution either with or without usual alternate immersion test in 9.3 per cent salt solution at 95 F. As an illustration of the method an extruded test bar of a magnesium alloy consisting of 4 per cent aluminum, 0.3 per .cent zinc. 0.2' per cent manganese, the balance being magnesium, was immersed for 10 minutes in a 28 per cent solution of hydrofluoric acid, then rinsed and dried. Upon subjecting the so treated article to the alternate immersion test in 3 per cent salt solution at 95 F., more than 20 per cent of the area of the specimen showed wrrosive attack in 4 days. A similar specimen immersed for 10 minutes in a 28 per cent solution of fluosilicic acid showed less than 12 per cent of corthe application of electrolysis. The protective 15 roded surface area upon subjection to the same coatings thereby produced, I have found. are subalternate immersion test for 4 days. Similar reject to considerable deterioration in time when suits were obtained upon similarly treating arexposed to either the atmosphere or aqueous alt ticles of other magnesium base alloys. solutions, particularly those of the nature of sea If d sir he tic m y be M i lly elecwater. I have now found that, if instead of the trolyzed while being Su d to e action of simple acid fluoride or soluble fluoride salts there e o ution s by pp i g a e ti l of 20 to 50 may be employed a complex acid fluoride, as for V s o to the article 1181118 cathode of example, hydrofluosilicic acid, a very superior magnesium. corrosion resisting coating is developed upon Other modes of applying the principle of the magnesium and its alloys. The invention, then, inv ntion m y be empl yed instead ofthose exconsists of the method hereinafter fully described I pl in d. hange being made 8 re ards the and particularly pointed out in the claims. method herein disclosed. provided the step 01' In carrying the invention into effect the arstepsstated by any of the following claims or the ticle of magnesium or an alloy thereof is imeq va of c Stated p p8 be mersed in an aqueous solution containing fluo- 3o p y silicic acid. The concentration of the solution I I her for P c y P n Out d disprefer to use is from about 20 to per cent, altinctly cl m 88 my inv n io though other concentrations may be used, such n method f flw ni fln as 10 per cent or less. The duration of the treat- Coating 011 the Surface of 31116188 01 ment in the solution will vary with its concennesium and i ys, t e p which mp ises tration and may be from about 5 to 15 minutes, immersing the article in a water solution of from although satisfactory results may be obtained about 10 to about 35 per cent of fluosilicic acid at with a longer or a shorter duration of treatment. 8 temperature below abbut for a time I normally prefer to immerse the article for about ci n o f rm 8 fl inK h reo 10 minutes in a 28 per cent solution of the acid at 4 In a method of forming ommsistant room temperature. Higher temperatures may be coating 0!! the Surface articles 01 memployed, although it is best to operate below nesium and its alloys, p ch co p ises about 40 C. to avoid excessive loss of silicon immersing the article in a water solution of from tetrafluoride which is evolved at higher temperou 1 o a out 5 pe ce t of fl i i ic a id at a atures. temperature below about 40' C. for a time sum- In comparison with the conventional method of treating solutions containing hydrofluoric acid, I have found that the resistance to corrosion of articles treated according to my improved method is as much as twice as great as determined by the cient to form a coating thereon, and subjecting the article while so immersed to an anodie electrolysis.

WILLIAM S. LOOSE. 

